Oxidation of lignin in microemulsion reactor
Basing on the constructed microemulsion system and the above characterizations, the lignin oxidation performance in the microemulsion system using water soluble CuSO4 as the catalyst was investigated (Table 2). In this study, the products from lignin oxidation are classified to p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) alcohol unit products according to their structure as it is widely accepted that lignin mainly contains these structural units.5 The oxidation of lignin in common solvents was conducted firstly for the comparison. The results show that the total yields of phenolic monomers are far from desired value when water, octane and n -propanol are used as the solvent correspondingly (Table 2, entries 1-3), where only 22.5 mg g-1 of phenolic monomers is produced in n -propanol, which can be ascribed to the limited solubility of lignin in n -propanol (Table 1, entry 3). Besides, the improved yields of phenolic monomers in the binary systems imply that the addition of octane or water ton -propanol can promote the oxidation of lignin (Table 2, entries 4 and 5). However, there is no obvious increase of phenolic monomers in the given ternary systems located in multiphase region (Table 2, entries 6 and 7), the limited solubility of lignin in these multiphase emulsions can be responsible for this situation. But interestingly, the yield of phenolic monomers in the microemulsion reaches to 90.2 mg g-1 (Table 2, entry 8, point c in Figure 3), which gives about 40 to 500 wt. % increment to those of above solvents, indicating that the formation of microemulsion can significantly intensify this lignin oxidation process. Additionally, the oxidation of lignin in the above microemulsion in the absence of catalyst was also studied (Table 2, entry 9). It gives 46.2 mg g-1 of phenolic monomers, but no products from H, G and S units are detected, which conforms both the thermal and the above solubilization effects during lignin depolymerization.
As mentioned, the highest yield of phenolic monomers is obtained from the above microemulsion (Table 2, entry 8, point c in Figure 3). Therefore, the mass ratio of n -propanol to octane in microemulsion was fixed for investigating the influence of WW (points a-f in Figure 3) during lignin depolymerization (Table 2, entries 8 and 10-13). It can be found that the yield of phenolic monomers increases significantly at first and then has a slight fluctuation with the increase of WW in the microemulsion region. For example, it is 32.6 mg g-1 in the absence of water, while it reaches to the maximum of 90.2 mg g-1 at the WW of 15.7 wt. % (Table 2, entry 8, point c in Figure 3). Incidentally, the yields of products from the transformation of H, G and S units reach to their peak values at the WW of 15.7, 24.4 and 31.2 wt. %, respectively (Table 2, entries 8, 12 and 13, points c-e in Figure 3). Whereas, there is an obvious decrease in phenolic monomer products when the WW further increases to 43.8 wt. % (Table 2, entry 14, point f in Figure 3). This decline can be attributed to the great decrease of interfacial area and the increase of mass transfer resistance in this multiphase emulsion compared with those in microemulsions. Similarly, the influence of WP in the microemulsion with the fixed RW/O of 1.71 (Line Ⅱ in Figure 3) on the lignin oxidation process was also investigated (Table 2, entries 8 and 15-17). The yield of phenolic monomers is only 59.7 mg g-1 at the WP value of 53.8wt. % (Table 2, entry 15, point g in Figure 3), which is partially because this ternary system locates in the multiphase region. Interestingly, the yield of phenolic monomers reaches to the peak value when the WP value increases to 75.1 wt. %, the same point as mentioned above. But it drops to 67.8 mg g-1 when the WP value further increases to 86.5 wt .% (Table 2, entry 18, point i in Figure 3). This can also be ascribed to the change of phase behavior and lignin solubility in the investigated system (Table 1, entry 6). It should be noticed that the constructed B.C and W/O microemulsions have similar efficiency (Table 2, entries 16 and 18, points h and j in Figure 3) for lignin oxidation compared with those of traditional solvent systems, while most of the given O/W microemulsions can promote the oxidation of lignin rationally. In belief, the type and component ratio of microemulsion are considered as the key factors for this oxidation process. But, it is interesting to point that the yields of products from the H unit are at least twice or triple to those from the G or S unit, although the ratio of H unit in lignin is the lowest, while the content of H, G and S units in bagasse lignin are 15, 45 and 40% respectively,41 demonstrating that these systems are prefer to oxidize H unit rather than other units.
Table 2. Effect of different solvents on the oxidation of lignin.